An MB-OFDM apparatus, also called a WiMedia UWB apparatus, can generate interferences towards other wireless systems, and particularly a WIMAX device that is a fixed wireless device (Worldwide Interoperability for Microwave Access). Such a WIMAX device operates, for example, with a bandwidth of 20 MHz at a central frequency of 3.5 GHz, whereas the frequency band of the MB-OFDM system lies between 3.1 and 10.6 GHz.
Wireless personal area networks based on OFDM and UWB technologies, like the MB-OFDM standard, will directly interfere with narrowband interferers that are close to such wide band devices. At present, no specific interference mitigation techniques are implemented in the UWB standard based on OFDM.
Orthogonal frequency-Division Multiplexing (OFDM) is a digital modulation method in which a signal is split into several narrowband channels (sub-carriers) at different frequencies. To avoid in-band spectral interference, a given attenuation level may be reached within a given transmission band of the interfering signal.
For example, one method is based on modulating (or zeroing) the transmitted sub-carriers located within the given transmission band in order to try to cancel the signal energy found in this given band.
However, because of the amplitudes of side lobes of the remaining transmitted sub-carriers (these side lobes are produced by the sin(x)/x shape spectrum of each transmitted sub-carrier), it is generally difficult to reach relatively high attenuation levels within a given band of the transmitted OFDM symbol.
In particular, in the coexistence perspective of MB-OFDM systems with other wireless systems, and particularly with WiMax, notches of as much as −30 dB with respect to the OFDM sub-carriers transmission power are requested.
Another type of method for obtaining relatively high attenuation levels within a given band of a transmitted signal is disclosed in “Active Interference Cancellation Technique for MB-OFDM Cognitive Radio,” Hirohisa Yamaguchi, 34th European Microwave Conference—Amsterdam, 2004. More precisely, according to this method, the band between sub-carriers s1 and s2 of an OFDM symbol is notched by 1) zeroing every sub-carrier between s1 and s2, and 2) modulating sub-carriers s1 and s2 using coefficients computed as a function of the OFDM symbol transmitted data, with the target of minimizing the energy in the band s1-s2. However, such a method is relatively complicated to implement.
Another method for removing sub-carriers within part of a transmission band to be notched is disclosed in European Patent No. 1,802,063 and is based, in particular, on filtering (either in frequency or time domain) the digital initial modulation signal with a frequency resolution higher than that of the initial modulation signal.